1) Field of the Invention
The present invention relates to cells using, as the cell cases, film outer casings (jackets) such as laminate films.
2) Description of the Related Art
Following a demand for a size reduction of mobile electronic appliances such as mobile phones and personal digital assistants, cells and batteries serving as the driving power sources of such appliances are required to be even thinner and lighter in weight.
In order to meet this demand, patent document 1 suggests a light-weight thin cell that uses, as the cell case, a film outer casing formed of a laminate film in which a metal foil, such as aluminum, and resin are laminated, and that has a flat electrode assembly housed in the casing.
Patent Document 1: Japanese Unexamined Application Publication No. 1999-224652.
This laminate film is of the structure that metal layers and resin layers are laminated, as shown in FIG. 5. As shown in FIG. 9, a laminate film having an electrode-housing depressed portion and a plate laminate film are bonded by heat compression or the like at four side edges (4a, 4b, 4c, 4d).
With this technique, an electrode-housing depressed portion for housing the electrode assembly therein is formed in advance, and thus no unnecessary space exists between the electrode-housing depressed portion and the electrode assembly, thereby providing more compact cells.
In recent years, with a growing need for further miniaturization of the cell, an attempt is being made to make shorter the distance between the flat electrode assembly housed in the cell outer casing and the sealed portion, thereby reducing unnecessary space. With this method, however, in the event that the flat electrode assembly is inclined when inserting it into the outer casing, side sealed portions (4b, 4c) are pressed while catching the electrode assembly, as shown in FIG. 8. This causes short circuiting between the electrode plate, which is the outermost layer of the electrode assembly 1, and a metal layer 100 of the laminate film (at the portion circled in the figure), and the metal layer 100 is thus polarized. This leads to corrosion of the metal layer 100, resulting in degradation of sealing performance.